1. Field of the Invention
The present invention relates to an X-ray tube high-voltage generating device using a converter circuit for rectifying and boosting an A.C. voltage.
2. Description of the Related Art
A typical prior art high voltage generator for use with an X-ray tube is shown in FIG. 1.
The generator comprises: an A.C. power source 2; a converter circuit 3 connected to receive an A.C. voltage from power source 2 and adapted for successive rectifying, chopper controlling, boosting and rectifying to produce a D.C. high voltage for application to an X-ray tube 4; and a feedback control circuit 15 for monitoring the direct-current high voltage produced by converter circuit 3 so as to control the switching action of converter circuit 3.
More specifically, converter circuit 3 comprises: an input rectifying circuit 6 for rectifying the A.C. voltage from power source 2; a boosting transformer T having a primary winding N1 whose neutral point is coupled to an output terminal (plus) of rectifying circuit 6 through a chopper transistor TR1 and whose both ends are alternately coupled to the other output terminal (minus) of rectifying circuit 6 through inverter transistors TR2 and TR3 and two secondary windings N2 and N3 across which A.C. high voltages are developed; output rectifying circuits 7a and 7b for rectifying the A.C. high voltages to provide D.C. high voltages; voltage dividing resistors R1 and R2 connected in series between positive and negative output terminals of rectifying circuit 7a; voltage dividing resistors R3 and R4 connected in series between positive and negative output terminals of rectifying circuit 7b, the positive output terminal of rectifying circuit 7b and the negative output terminal of rectifying circuit 7a being connected together; a tube-current detecting circuit 8 coupled to the positive output terminal of rectifying circuit 7a, which is connected to the anode of X-ray tube 4; and a cathode heating circuit 9 connected between the negative output terminal of output rectifying circuit 7b, which is connected to a terminal of a cathode of X-ray tube 4, and the other terminal of the cathode of X-ray tube 4.
Tube-current detecting circuit 8 is adapted to detect a tube current of X-ray tube 4 and control cathode heating circuit 9 in accordance with the result of the tube-current detection. As a result, the temperature of the cathode of X-ray tube 4 is so controlled that the tube current has a desired value. Broken lines in FIG. 1 denote control signal lines.
Feedback control circuit 15 comprises: a tube-voltage detecting circuit 10 connected to a junction of resistors R1 and R2 and a junction of resistors R3 and R4 and adapted to detect the D.C. high voltage impressed across X-ray tube 4 through detection of a voltage obtained by dividing the D.C. high voltage by means of resistors R1 through R4; and a chopper control circuit 11 adapted to control chopper transistor TR1 in accordance with the tube voltage detected by tube-voltage detecting circuit 10.
In the prior art circuit, chopper control circuit 11 always controls the on-time (duty factor) of a switching pulse applied to chopper transistor TR1 in accordance with the detected tube voltage. As a result, even if the A.C. voltage of power supply 2 varies, the tube voltage will be kept at the desired constant value by means of the feedback control. For this reason, the prior art circuit will suffer disadvantages at the start of operation. That is, when the operation starts, a tube voltage is very small or almost 0 volts. This means that a detected tube voltage and the desired voltage differ significantly. This results in a much increased on-time of the switching pulse produced by chopper control circuit 11. As a result, the voltage and current of chopper transistor TR1 and inverter transistors TR2 and TR3 could be twice as large as the normal voltage and current. Therefore, these transistors must have high ratings in order to avoid overload.
In order to obviate the disadvantages, an attempt has been made to control the chopper transistor by a switching pulse with a fixed pulse width (duty ratio) during a certain time from the start of operation without performing the feedback control and to subsequently perform the above-described feedback control. In this case, however, an output D.C. high voltage can have degraded rising characteristics. For example, it may take a long time for the output D.C. voltage to rise under a high output condition (high voltage and high current), while an overshoot may occur in the output D.C. voltage under a low output condition.
As described above, the problems with the prior art device are that the transistors in the converter circuit may be overloaded, or the rising time of the output D.C. voltage may be prolonged or the overshoot may occur in the output voltage depending on the output conditions.